BTC is a company (I assume) and I represents a customer.
The major values in this problem are...
The budget ( which is be our Y): 20 bucks aka 2000 cents
The rate to make a call per out going call (which is our slope aka M): 40 cents
The amount of times the customer calls (Which will be our X): not found yet aka ????cents
so the equation is 2000=40X
SOLVING FOR X gives us the number of times the customer can call.
so x=50 calls
Tips for solving these:
Think about the role that each variable serves and how you can represent that using one of the equations you know. Also memorize the basic expressions such as Mx+b and X^2 and what they can be used for and what they look like. Often times finance problems are just linear or exponential. If your class has not covered exponential, your best bet is to just think linear unless it notes other wise. If you need anymore help contact me, I'll be available all evening :)